The Golden Rule transcends time and culture. Social reciprocity, as implied by the Golden Rule, is a crucial component of normal social interactions. Individuals with psychiatric disorders marked by social deficits often fail to reciprocate normally with others. Although various types of reciprocity have been observed in humans and non-humans, the underlying neural mechanisms remain unknown. An animal model of complex social cognition that could be explored with single-unit recording and pharmacological manipulations of specific groups of neurons will help uncover the neural mechanisms underlying social reciprocity. Here we propose to develop a new social game in interacting dyads of rhesus monkeys. We have previously shown that rhesus monkeys prefer to donate juice rewards to other monkeys when the alternative is to reward neither monkey, but prefer to reward just themselves when the alternative is to share the rewards with others. In the new social game proposed here, the reward-donating (prosocial) and reward-withholding (antisocial) preferences displayed by one monkey could be subsequently reciprocated by the other monkey in a turn-taking fashion. This game will be used to investigate how neurons in the primate prefrontal cortex are involved in the computation of reciprocity during social interactions. We will record neuronal spiking activity from a brain structure in the medial prefrontal cortex called the anterio cingulate gyrus (ACCg), as it contains neurons that signal both the rewards received by oneself and another individual. In particular, we will investigate whether, how, and at what points in time the reciprocity-induced changes in social preferences are reflected in the activity of ACCg neurons. We hypothesize that ACCg activity associated with rewards received by self and others is gain modulated by the reciprocity level across two interacting monkeys. Furthermore, we will examine how the activity in one monkey's brain is dynamically modulated with respect to the neuronal activity in the other's brain during these reciprocal reward exchanges. We hypothesize that the reward outcome signaling by the ACCg is central to reciprocity, endorsing the notion that ACCg belongs to the core circuits mediating normal social behaviors.